Division for Diagnostics and Scientific Advice, National Veterinary Institute, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark.
Division for Diagnostics and Scientific Advice, National Veterinary Institute, Technical University of Denmark, 2800 Kgs. Lyngby, Denmark; Department of Veterinary and Animal Sciences, Faculty of Health and Medical Sciences, University of Copenhagen, 1870 Frederiksberg, Denmark.
Prev Vet Med. 2019 Nov 1;171:104689. doi: 10.1016/j.prevetmed.2019.05.002. Epub 2019 May 8.
Bacterial culture (BC) and quantitative polymerase chain reaction (qPCR) are widely used as diagnostic tests for pathogens causing intra-mammary infections (IMI) and it is therefore important to evaluate their performance to optimize pathogen detection. However, with no reference standard, their sensitivity (Se) and specificity (Sp) are unknown. Furthermore, the Se and Sp can differ between quarter samples and composite samples, both of which can be used to test for IMI. Latent class analysis (LCA) offers a method for estimating the Se and Sp of two tests on the same population, but to our knowledge this has not yet been applied to qPCR and BC for the detection of Staph. aureus in both composite and aseptically collected quarter milk samples. This allows for a performance evaluation of not only the two diagnostic methods, but also the two different samples -both cow and quarter level. In this study, we used Bayesian LCA on a dataset from one sampling day at a Danish dairy herd to estimate the Se and Sp for detecting Staph. aureus at cow and quarter level. We used Ct cut-offs of 32 and 37 cycles for the qPCR. When using a cut-off of 37 cycles, the estimated Se and Sp for BC were 62.2% and 90.5% at cow level, and 90.7% and 93.5% at quarter level, respectively. Similarly, the estimated Se and Sp for qPCR at cow level were 82.7% and 84.1%, respectively, and 80.0% and 96.8% at quarter level. Sp was therefore higher for both BC and qPCR at quarter level. Se was also highest for BC at quarter level, but the opposite was true for qPCR. The same pattern was found using a Ct cut-off at 32 cycles, but with different estimates. The results show that qPCR with a Ct cut-off at 37 had a higher Se than BC for composite DHI samples and it is therefore more suitable as a routine screening test for Staph. aureus. However, BC on quarter samples gave the highest Se and Sp and should therefore be used for confirmatory testing.
细菌培养 (BC) 和定量聚合酶链反应 (qPCR) 被广泛用作引起乳腺炎 (IMI) 的病原体的诊断测试，因此评估它们的性能以优化病原体检测非常重要。然而，由于没有参考标准，因此它们的敏感性 (Se) 和特异性 (Sp) 是未知的。此外，四分体样本和混合样本的 Se 和 Sp 可能不同，这两种样本都可用于 IMI 检测。潜在类别分析 (LCA) 提供了一种在同一人群中对两种测试进行估计 Se 和 Sp 的方法，但据我们所知，这尚未应用于 qPCR 和 BC 检测复合和无菌采集的四分体牛奶样本中的金黄色葡萄球菌。这允许不仅对两种诊断方法，而且对两种不同的样本进行性能评估 - 包括奶牛和四分体水平。在这项研究中，我们在丹麦奶牛场的一个采样日的数据集上使用贝叶斯 LCA 来估计在奶牛和四分体水平上检测金黄色葡萄球菌的 Se 和 Sp。我们使用 qPCR 的 Ct 截止值 32 和 37 个循环。当使用 37 个循环的截止值时，BC 的估计 Se 和 Sp 在奶牛水平上分别为 62.2%和 90.5%，在四分体水平上分别为 90.7%和 93.5%。同样，qPCR 在奶牛水平上的估计 Se 和 Sp 分别为 82.7%和 84.1%，在四分体水平上分别为 80.0%和 96.8%。因此，BC 和 qPCR 在四分体水平上的 Sp 更高。BC 在四分体水平上的 Se 也最高，但 qPCR 则相反。使用 32 个循环的 Ct 截止值也发现了相同的模式，但估计值不同。结果表明，对于复合 DHI 样本，使用 Ct 截止值 37 的 qPCR 的 Se 高于 BC，因此更适合作为金黄色葡萄球菌的常规筛选测试。然而，四分体样本上的 BC 提供了最高的 Se 和 Sp，因此应用于确认测试。
